Recently, with a trend of higher density and expansion of applications, a storage battery (secondary battery) including a lithium ion battery is used in portable communication equipment such as a laptop computer, a mobile phone and the like, a hybrid/electric automobile or even in a storage battery for household. Demands for reduction of energy consumption and reduction of power consumption have risen with the recent environmental problems and the like as a background, and use of a storage battery is expected to be able to respond to the request. However, in order to utilize the storage battery for reduction of power consumption, power amount management (battery capacitance management) of the storage battery is an important problem.
Conventionally, the power amount management of the storage battery has been made by estimating and managing/controlling a charge/discharge amount by using experimental formulas and the like including time and temperature information on the basis of a terminal voltage of the storage battery.
However, in the case of the conventional method, an electricity storage condition of a storage battery (change of a battery capacitance) is estimated indirectly on the basis of the terminal voltage of the battery by considering time/temperature information and thus, the estimation accuracy is limited, and over-charging or insufficient charge might occur depending on an operating situation or a use environment condition. Moreover, a problem of over-discharging or the like also occurs in discharge (when the battery is in use) with the conventional method.
Moreover, a flammable material is used in a storage battery in many cases, and a problem in safety such as a risk of ignition caused by over-charging is also pointed out. Moreover, if a user does not accurately recognize insufficient charge of a storage battery, the user might feel inconvenience about a charge timing earlier than expected. On the other hand, over-discharging reduces a performance life of a battery due to a characteristic of a battery. Moreover, if a storage battery has indication of a remaining amount, if the remaining amount becomes small, drop of a terminal voltage becomes rapid due to the battery characteristic, and as a result, a sense of a “dead battery” comes earlier than expected and not match an actual feeling of use, which is inconvenient.
The above-described problems are caused by inability of accurate detection of a change in a battery capacitance in charge/discharge depending on an operating situation or a use environmental condition of a secondary battery.